Robert Gendler
Astrophotographer & Physician
Fellow: Royal Photographic Society
Author: A Year in the Life of the Universe: 
A Seasonal Guide to Viewing the Cosmos

How will humanity first discover ET Life?
Best guess: Biosignatures
Simple “Prokaryotic type” life: High probability, Biosignatures
Complex Life: Very low probability
Intelligent (technological) Life: Dismal probability

On the existence of Extraterrestrial life 
Since 1992, 4260 confirmed exoplanets discovered
Discovery of exoplanets has raised excitement and optimism among scientists and the lay public about the existence of extraterrestrial life (ET) in our local universe. 
Scientists who seem most optimistic are invariably physical scientists (astronomers and astrophysicists). 
When the same question of ET life is proposed to evolutionary biologists they invariably do not share the same optimism.

On the existence of Extraterrestrial life 
Physical scientists are trained to think deterministically and apply that same reasoning to all natural phenomenon. 
Physical Determinism dictates that all natural phenomenon should be reducible to predictable equations and thus be repeatable given the same starting conditions. 
Biological Systems are inherently NOT deterministic, but rather opportunistic. 
Life evolving on Earth does not imply Life is ubiquitous in the Universe
Evolutionary biologists understand the immense improbability of complex life evolving even on earth

Factors favorable to the origin and sustained existence of life on Earth
The earth possesses liquid water in abundance, a likely requirement for life. 
Earth possesses a stable atmosphere which provides protection to living organisms from harsh UV radiation. 
Earth has sufficient density to hold onto that atmosphere. 
Earth possesses a magnetosphere as a result of its unique molten liquid core (unique to earth in our solar system).
Rotating molten liquid core is responsible for earth's dynamic crust producing plate tectonic activity, a major driver of evolution during the vast history of life on earth. 
Earth's magnetosphere protects surface life from the ionizing solar wind. As far as we know a magnetosphere may be a unique property of our planet.

Factors favorable to the origin and sustained existence of life on Earth
Earth has a planet sized moon which likely absorbed countless meteor impacts which might otherwise have created havoc and a clear danger to emerging life on our planets young surface. 
Our solar system has several enormous gas/ice giant planets which likely protected the early earth from large devastating impacts during the period of bombardment. 
We have a single sun (half of all stars exist in binary systems).
Our sun is just the right mass to deliver a sustained supply of energy to support life over billions of years.

Factors favorable to the origin and sustained existence of life on Earth
Our sun is just the right distance from earth to nurture life without scorching our planet's surface nor rendering earth a frozen wasteland.
Our sun is stable…..not an unstable variable star 
Our solar system formed from the debris of a much earlier supernova filling the interstellar medium with heavy elements necessary for life. (we have a high metallicity sun)

Summary of earth properties favorable to life
Liquid surface water
Sufficient density/size to prevent oceans from boiling off
Atmosphere to protect life from harmful UV radiation harmful to life
Magnetosphere that prevents harmful effects of ionizing solar wind radiation
Rotating Liquid Iron core produces a rotating crust (plate tectonics) a strong driver of evolution over billions of years
Single non-variable sun (half of all stars are binaries)
Sun has stability, proper mass, and at proper distance to sustain life over billions of years (Goldilocks Zone)
Earth’s planet sized moon and our solar system’s gas giants may have protected the early earth from catastrophic bombardment harmful to early life
Our solar system formed from heavy element rich substrate necessary for life (our sun has high metallicity

Realistic probability of extraterrestrial simple life
Extraterrestrial simple life (ie; prokaryotic life) is likely but not to the extent physical scientists predict. 
Evolution is an opportunistic process characterized by occasional progress, long periods of quiescence, extinctions, mass extinctions, infinite chance factors, with no particular goal or direction. Complex life took 4 billion years to occur on earth. This required a most highly improbable stability of earth, our sun, our solar system, our local galactic environment. 
Therefore the multiple origin of simple self replicating nucleic acid-protein aggregates (prokaryotic type life)is likely high

Realistic probability of extraterrestrial complex (multicellular) life
It is most certain that prokaryotic life arose rather quickly after the formation and cooling of our planet sometime around 4 billion years ago. 
For the next 1.5 billion years photosynthetic bacteria changed earth's atmosphere to an oxygen containing one. At the cellular level nothing much else happened. 
Then a most improbable event occurred. One prokaryotic cell engulfed another and began a symbiotic relationship creating the Eukaryotic cell. 
For the next nearly two billion years living cells underwent several evolutionary experiments, one of which was sexual reproduction. Sexual reproduction dramatically drove biodiversity and evolution however billions of years passed before complexity evolved. 
So for over 3.5 billion years (greater than half the age of the earth) life continued to exist solely as single celled organisms in earth's vast ocean. The land remained sterile for the first 90 percent of life's existence on earth. 
The prevalence of complex (multicellular) life in the universe is therefore very low.

Realistic probability of extraterrestrial Intelligent (technological) life
Approximately 536 million years ago, (almost 3.5 billion years after life's first emergence on earth) 
cells were ready to assemble into more complex organisms. It still took another 534 million years 
for our early hominid ancestors to evolve.
Before that could happen life had to struggle through several major evolutionary experiments in, amphibians, reptiles, mammals, primates, hominids. 
Life also had to sustain itself through five major mass extinctions, one of which nearly sterilized the planet. 
It was a mere 200 thousand years ago that anatomically modern human beings began to walk the earth.
Only 150 years ago our species began to produce high technology. 
Four billion years have passed and over a trillion species have come and gone and only one of 
those species has produced technology (and only in the last .0005 % of its time as a species). 
This sobering realization has to render the romantic expectation of ubiquitous intelligent life 
in the universe a very naive and unlikely one.

Realistic probability of communicating with extraterrestrial Intelligent beings
Quote from the great evolutionary biologist Ernst Mayr.....
“Even if there were intelligent extraterrestrial life, and even if it had developed 
a highly sophisticated technology, the timing of their efforts and those of our 
engineers would have to overlap to an altogether improbable degree, considering 
the amounts of astronomical time and space available. Every aspect of 
“extraterrestrial intelligence” that we consider confronts us with astronomically 
low probabilities. If one multiplies them together, one comes out so close to zero, 
that it is zero for all intents and purposes.”

So in summary there are likely other planets having some or all of the favorable attributes 
of earth suitable to producing simple self replicating nucleic acid-protein aggregates 
(prokaryotic type organisms) but certainly much fewer than most physical scientists assume. 
A planet capable of producing life must also have the various properties and fortunate 
circumstances to sustain it over billions of years for evolution to run its course and 
lead to complex multicellular life. 
I believe it is doubtful there are many exoplanets with such a fortunate combination of 
favorable factors to sustain life over the billions of years needed to achieve higher 
complexity multicellular life. 
Intelligent extraterrestrial life is most certainly an extremely rare occurrence and 
quite possibly a unique one limited to Earth.

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